1. Field of the Invention
This invention relates to video signal processing apparatus for use with a video camera and, more particularly, to such apparatus which, in one embodiment, inserts into the video signal derived from that camera a black signal information component to which other video processing circuits may be clamped; and which, in another embodiment, produces an improved contour-emphasized luminance signal.
2. Discussion of the Prior Art
In video image pickup apparatus, such as television cameras (for example, television cameras having charge coupled device pickup members), still video cameras and video signal processing circuits used with such cameras, several stages of processing circuitry carry out operations which are dependent upon a reference video signal level, such as that which represents black information. The accuracy with which such signal processing operations are performed is dependent upon a reference black level which best represents the optical black level that may be derived from the image pickup device of the camera, even if the scene being imaged does not then contain black information. A typical video camera, whether of the CCD-type or otherwise, scans information imaged onto a pickup element in a line-by-line raster array formed of a line scanning period, during which useful video information is scanned, followed by a flyback period, during which the video camera is conditioned to scan the next line of image information. Usually, information representing a desired black reference signal level is inserted into the video signal during the flyback period. Luminance and chrominance components which are derived from the image information produced by the pickup element are formed relative to this reference black signal level.
One example of a CCD imaging arrangement in which a black reference signal level is inserted into the horizontal flyback period of the video signal produced by a television camera is illustrated in FIG. 1. Here, the image pickup element is represented as a CCD device 2 having a useful image pickup area 2A and light-shielded optical black surface 2B adjacent the image pickup surface. As is conventional, electrical signals produced as stored charges across CCD device 2, including the signals produced across image pickup surface 2A and optical black surface 2B, are shifted on a line-by-line basis into a horizontal storage register 2C from which the contents are read out in serial fashion. The resultant photoelectric output S.sub.1 from horizontal register 2C is amplified and supplied to an output terminal P.sub.1 as an image pickup signal S.sub.2.
A schematic representation of the image pickup signal S.sub.2 is illustrated as the waveform shown in FIG. 3A. This image pickup signal is comprised of useful video information W.sub.1 produced during a line scanning period T.sub.1, followed by the flyback period formed of portions T.sub.2, T.sub.4 and T.sub.3. The sum of these periods T.sub.1 -T.sub.4 is equal to a horizontal scanning interval, known as a "1 H" interval. During portion T.sub.2 of the horizontal flyback period, the signal W.sub.2 derived from the scanned optical black surface 2B of FIG. 1 is generated. The level of this signal W.sub.2 is the black reference signal level produced by CCD device 2 and is equal to the video information produced when a truly black image is scanned.
When image pickup device 1 of FIG. 1 is used with other signal processing circuitry of the type normally included in a video camera, the image pickup signal S.sub.2 produced by the image pickup element is supplied to a processing circuit, such as circuit 4 shown in FIG. 2. The image pickup signal S.sub.2 is sampled by a correlated double sampling circuit 5 (known as a CDS circuit), as schematically represented in FIG. 3A. The sampled signal is subjected to automatic gain control by an AGC circuit 6 to produce an image signal S.sub.3 having the waveform shown in FIG. 3B. Here, the image signal S.sub.3 is comprised of useful video information W.sub.11 during line scanning period T.sub.1, and the black reference signal level W.sub.12 derived from optical black surface 2B during portion T.sub.2 of the horizontal flyback period. The black reference signal level W.sub.12 is referred to as the optical black signal level. The image signal also includes a blank signal W.sub.14 during portion T.sub.4 of the flyback period, as well as a blank signal W.sub.13 during that portion T.sub.3 of the flyback period which immediately precedes scanning period T.sub.1.
The image signal S.sub.3 shown in FIG. 3B is applied to a clamp circuit 7. This clamp circuit is provided with a clamp pulse CLP.sub.1, shown in FIG. 3C, during portion T.sub.2 of the flyback period to clamp the signal level of the image signal during portion T.sub.2 to the optical black signal level W.sub.12 which then is present in the image signal S.sub.3. Clamp circuit 7 supplies to a signal processing section 8 the clamped image signal S.sub.4 (whose waveform is similar to that of signal S.sub.3 in FIG. 3B) with the optical black signal level W.sub.12 properly clamped to the level derived from the scanning of optical black surface 2B of CCD pickup 2. The signal processing section utilizes the clamped optical black signal level W.sub.12 as a reference from which various video processing operations are carried out. The resultant processed video signal S.sub.5 is supplied to an output terminal P.sub.2 by signal processing section 8.
Typical processing operations that may be carried out by signal processing section 8 include gamma correction, luminance signal derivation, chrominance signal carrier balance, 1 H delay circuit gain adjustment, aperture control and horizontal synchronizing signal generation. These operations typically rely upon a fixed, unvarying reference signal level that is related to the image signal produced by image pickup device 2. By using the optical black signal level W.sub.12 derived from optical black surface 2B, a proper reference signal level is obtained. However, signal processing section 8 normally is formed of a number of individual operating stages, each adapted to carry out a respective signal processing operation. In doing so, each stage establishes its own reference signal level in order to execute its designated operation; and this is done by clamping the optical black signal level W.sub.12 which had already been clamped by clamp circuit 7.
As each stage carries out its own individual clamping operation, inconsistencies and/or errors may be introduced into the signal level which ultimately is clamped. That is, although the optical black signal level W.sub.12 present in clamped image signal S.sub.4 may be accurate, subsequent clamping operations carried out on this optical black signal level may be accompanied by noise, which results in an inaccurate noise-induced clamped reference signal level for a particular stage. In another stage, the clamped reference signal level may include a leakage voltage from the clamping pulse itself, which deforms the reference signal level in that stage. Consequently, the reference level which should be uniform in all stages may vary from one stage to another because of inaccuracies introduced by the individual clamping operations that are carried out by each such stage. The likelihood of such deviations in the reference level increases as the number of signal processing stages and, thus, the number of individual clamping operations, increases. Hence, errors may develop because signal operations which are carried out with the expectation that they all are referenced to the same reference level are, in fact, performed on the basis of different reference levels. Clearly, this is a drawback and disadvantage attending known video cameras and the video signal processing circuits used with those cameras.
Some color video cameras include an optical complementary color matrix filter through which the optical image of a scene is projected to the image pickup element. A small portion of such a complementary color matrix filter is schematically illustrated in FIG. 4 as including successive rows of alternating color filter elements. For example, one row is comprised of alternating yellow and cyan filter elements Y.sub.E, C.sub.Y, Y.sub.E, C.sub.Y, . . . , etc., and the next row is formed of alternating magenta and green filter elements M.sub.G, G, M.sub.G, G, . . . , etc. Thus, the complementary color matrix filter is comprised of one row formed of alternating Y.sub.E and C.sub.Y elements followed by the next row formed of alternating M.sub.G and G elements, and so on. Typically, the photoelectric signals which are produced by the image pickup element in response to a visual scene being imaged through the matrix filter are scanned such that those signals produced in response to the imaging through two adjacent rows of filter elements are scanned simultaneously. Thus, when a CCD image pickup element is used, the charges stored as a result of imaging through two vertically adjacent rows are scanned simultaneously when one horizontal raster line of image signals is produced.
Often, the patterns of the adjacent rows of filter elements from which an even scan line of image signals is produced differ from the patterns of the adjacent rows of filter elements from which an odd scan line of image signals is produced. This difference generally is equal to one row of filter elements. Accordingly, and with reference to FIG. 4, when an even horizontal line is scanned, that is, when an even scan line of image pickup signals is produced, that scan line may be derived from color filter elements Y.sub.E +M.sub.G, C.sub.Y +G, Y.sub.E +M.sub.G, C.sub.Y + G, etc. However, when an odd line is scanned, the image pickup signals produced by that odd scan line may be derived from the color filter elements Y.sub.E +G, C.sub.Y +M.sub.G, Y.sub.E +G, C.sub.Y +M.sub.G, etc.
It is appreciated that red and blue color signals may be formed of predetermined arithmetic combinations of yellow, cyan and magenta signals. For example, the red color signal may be formed by subtracting a cyan signal from the sum of yellow and magenta signals. As another example, the blue color signal may be formed by subtracting a yellow signal from the sum of magenta and cyan signals. Furthermore, the luminance signal Y is a function of the average value of the image pickup signal derived from all of the filter elements of the complementary color matrix filter. Thus, by arithmetic combination of the yellow, cyan and magenta signals, red and blue color difference signals R-Y and B-Y, as well as the luminance signal Y may be obtained.
However, it is known that, for a given brightness of a visual scene, the signal level derived from a yellow filter element Y.sub.E is greater than the signal level derived from a magenta filter element M.sub.G which, in turn, is greater than the signal level derived from a cyan filter element C.sub.Y, which is greater than the signal level derived from a green filter element G. Because of this difference in the signal levels, the modulation component produced by the filter elements when an even line is scanned differs from the modulation component that is produced when an odd line is scanned. This difference in the modulation levels is represented by the waveforms shown in FIGS. 5 and 6.
FIG. 5 represents the image pickup signal produced when an even line is scanned and is derived from the photoelectric signals produced by filter elements Y.sub.E +M.sub.G, C.sub.Y +G, Y.sub.E +M.sub.G, C.sub.Y +G, etc. FIG. 6 represents the image pickup signal produced when an odd line is scanned and is derived from the photoelectric signals produced by filter elements Y.sub.E +G, C.sub.Y +M.sub.G, Y.sub.E +G, C.sub.Y +M.sub.G, etc. For convenience, FIGS. 5 and 6 also illustrate a saturation voltage level E.sub.SAT, which represents the level of the image pickup signal whereat signal processing circuitry to which the image pickup signal is supplied operates in saturation. It is appreciated that, for proper signal processing, the level of the image pickup signals should be less than this saturation voltage level E.sub.SAT. Although the modulation swings of the image pickup signals produced during the even and odd line scans differ (in the example illustrated, the modulation component produced during even line scans in greater than the modulation component produced during odd line scans), it is assumed that the signal level of the even and odd scan lines does not exceed the saturation voltage level E.sub.SAT and, therefore, the average value of these image pickup signals is substantially equal.
If the brightness of the object being imaged increases, the modulation swings of the image pickup signals likewise increase, as illustrated in FIGS. 7 and 8. Consistent with FIGS. 5 and 6, FIG. 7 represents the image pickup signal produced when an even line is scanned and FIG. 8 represents the image pickup signal produced when an odd line is scanned. In the example being described, the brightness of the object being imaged is such that the level of the image pickup signals produced during an even scan line exceeds the saturation voltage level E.sub.SAT. If the maximum level of the image pickup signal produced during an odd scan line remains less than this saturation voltage level, then the level of the average value E.sub.AVL of the image pickup signals produced during an even scan line will be less than the level of the average value E.sub.AVL of the image pickup signal produced during an odd scan line. The fact that these average values differ from each other deleteriously influences a contour correction operation that normally is carried out on the image pickup signals derived from the video camera.
Contour correction in the vertical direction typically is performed by extracting components of adjacent lines of the luminance signal which are not correlated with each other. Such uncorrelated components typically are present at a contour; and these extracted components are emphasized and used to provide contour correction of the luminance signal. From the preceding discussion, it is recognized that the luminance signal is related to the average value of the image pickup signals derived from the complementary color matrix filter. Thus, if the levels of the average values of the image pickup signals produced during the even and odd scan lines differ, such as shown in FIGS. 7 and 8, the uncorrelated components in successive lines of the luminance signal will be much larger than they should be. In contour correction, the extracted, uncorrelated components are superimposed on the luminance signal. Hence, when the difference in the levels of the average value E.sub.AVL of the image pickup signals produced during even and odd scan lines is large, erroneous contour correction is performed, resulting in degraded quality of the resultant video picture.
It has been proposed, such as in Japanese Laid-Open Patent Application No. 269873/88, to eliminate this difficulty by suspending the contour correction operation if the video signal processing circuits are saturated. However, this requires a rather intricate detecting circuit and adds to the overall complexity of the video signal processing circuitry. Furthermore, if contour correction is suspended when saturation is detected, that is, when the brightness of the object being imaged is relatively high, an abrupt change in picture quality may be observed. FIG. 9 is a graphical representation of an abrupt suspension in the contour correcting operation when the brightness of the scene being imaged exceeds a value which would result in saturation of the signal processing circuitry.